A silica glass is used for a lens, a prism and a photomask of a photolithography instrument in manufacturing of a large-scale integrated circuit (LSI), for a TFT substrate used for display, for a tube of a ultraviolet lamp or an infrared lamp, for a window material, for a reflection plate, for a cleaning container in a semiconductor industry, for a container for melting of a silicon semiconductor, and so forth. However, an expensive compound such as silicon tetrachloride must be used as a raw material for a silica glass used in these silica articles; on top of that, melting temperature and processing temperature of a silica glass is extraordinary high (about 2000° C.), thereby leading to a high energy consumption, and this in turn to causing enormous emission of carbon dioxide, which is considered to be one of the global warming gases. Accordingly, from the past, a method for producing a silica glass at relatively low temperature by using a relatively cheap raw material has been sought.
For example, in Patent Document 1, a method (sol-gel method) in which a silicon alkoxide is hydrolyzed to a silica sol, which is then gelated to a wet gel, then to a dry gel by drying, and finally to a transparent silica glass body by firing at high temperature is disclosed. In Patent Document 2, a method in which a transparent silica glass is obtained by a sol-gel method from a silica sol mixture solution formed of tetramethoxy silane or tetraethoxy silane and a silica sol solution containing silica fine particles is disclosed. In Patent Document 3, a method for producing a transparent silica glass by using a silicon alkoxide and silica glass fine particles as its main raw materials, wherein a heating process at the temperature range of 200 to 1300° C. is carried out under an oxygen gas-containing atmosphere, a further heating process to 1700° C. or higher is carried out under a hydrogen gas-containing atmosphere, and a heating process between the foregoing two heating processes is carried out under a reduced pressure atmosphere, is disclosed. In these sol-gel methods, however, not only there were problems in a dimensional precision and a durability of the produced silica glass but also production cost thereof was not so cheap.
In Patent Document 4, a method (slip casting method), wherein at least two different kinds of silica glass particles, for example, silica glass fine particles and silica glass granules are mixed to obtain a water-containing suspension solution, which is then press molded and sintered at high temperature to obtain a silica-containing composite body, is disclosed. In Patent Document 5, a method, wherein a mixed solution (slurry) containing silica glass particles having the size of 100 μm or less and silica glass granules having the size of 100 μm or more is prepared, then the slip is cast into a molding frame, dried, and then sintered to obtain an opaque silica glass composite material, is disclosed. In these conventional slip casting methods, however, shrinkage of a molded article in drying and sintering processes is so large that a thick silica glass article with a high dimensional precision could not be obtained.
Accordingly, there are problems as mentioned above in the method for producing a silica glass article from a powdered raw material. Therefore, as a method for producing a silica crucible for manufacturing of a single crystal silicon used for LSI, methods disclosed in Patent Document 6 and Patent Document 7 are being used still today. In these methods, after a powdered natural quartz or a powdered synthetic cristobalite, which are ultra highly purified, is fed into a rotating mold made of carbon and then molded, carbon electrodes are inserted from the top and then electrically charged to cause arc discharge thereby raising the atmospheric temperature to a temperature range for melting of the powdered quartz (estimated temperature in the range from about 1800 to about 2100° C.) so that the powdered raw quartz may be melted and sintered.
In these methods, however, there has been a problem of a high cost because a highly purified powdered quartz is used. In addition, when a silica crucible is produced by a conventional method for manufacturing of a silica glass article and then used for growing of a silicon single crystal, there has been a problem such that much of gaseous bubbles are incorporated into a grown silicon single crystal.
In Patent Document 8, a silica crucible formed of three layers of an outer layer comprised of a natural quartz glass, an intermediate layer comprised of a synthetic quartz glass containing aluminum in high concentration, and an inner layer comprised of a high purity synthetic quartz glass, obtained from a powdered silica raw material by an arc-discharge melting method is described (the melting seems to be carried out under an air atmosphere). In it, prevention effect of impurity migration by the intermediate layer is shown.
However, this prevention effect of impurity migration is small; accordingly, when an alkali metal element such as Li, Na, and K is contained in high concentration in the outer layer, it has been difficult to suppress diffusion of these elements significantly.
In Patent Document 9, a technology to reduce gaseous bubbles in a wall of a melted quartz crucible by aspiration from a peripheral of a molding frame at the time of an arc-discharge melting of a molded article of a powdered silica raw material is shown.
However, dissolved gases in a wall of a melted quartz crucible could not be removed completely by mere aspiration of an air present in the powdered silica. It was only possible to produce a crucible containing a large quantity of residual gases, in particular, O2 and H2O.
In Patent Document 10, a silica crucible formed of three layers containing an inner layer comprised of a semitransparent quartz glass produced by an arc-discharge melting method is shown.
However, when a single crystal silicon is pulled up by using this three-layered crucible, there has been a problem of forming a defect such as a void and a pinhole in the pulled-up single crystal silicon and the other problems.